Symbol synchronization (timing recovery) and frequency offset correction (carrier recovery) are two of the most critical receiver functions in synchronous communications systems. In systems of this type, the receiver clock must be continuously adjusted in frequency and phase in order to optimize the sampling instants of the data signal recovered from each received signal. The choice of sampling time is critical for minimization of the error probability due to intersymbol interference and noise, particularly when the received signal is subjected to sharp roll-off filtering during the process of recovering the data signal from it.
To avoid performance degradation caused by carrier phase and frequency error, the incoming signal carrier must be tracked. The timing and carrier frequency information are often derived from the recovered data signal itself, based on some meaningful optimization criterion which determines the steady-state location of the timing instants and received carrier frequency.
Digital radio receivers often employ circuitry for tracking the carrier frequency of a received signal, and automatically correcting for discrepancies in between the carrier frequency and the frequency of a local oscillator in the receiver (e.g., a local oscillator used for superheterodyne reception). They also employ timing recovery circuitry to enable correct recovery of digital data conveyed by each received signal.
In mobile communication environments in which a modulated carrier is subject to fast multi-path fading, most conventional continuous time synchronization techniques cannot perform satisfactorily. In such environments, the most practical synchronization techniques are those which derive synchronization information from the sampled, recovered data. In time domain multiple access systems in which each user is assigned a time slot, the receiver must perform its timing and frequency synchronization task on a slot by slot basis. Because such systems usually employ automatic frequency control circuitry to perform frequency synchronization (and such automatic frequency control circuitry have inherent frequency error), the timing recovery circuitry employed must accommodate at least a limited amount of frequency error without significant degradation in performance.
There are several types of conventional timing recovery techniques. Examples of such techniques include comparison of the threshold crossings of recovered base band data with the sampling phase, measurement of the phase of narrow-band-pass filtered base band data (where the narrow band-pass filter is centered at a receiver clock frequency), "differentiate and multiply" techniques which generate an error signal (during each sample interval) proportional to the product of the time derivative of the base band signal at each sampling time and the polarity of the base band signal at such time, and techniques which employ a bank of all-pass filters to estimate timing offset of a base band signal having a frame structure which contains a synchronization field.
There are also several conventional automatic frequency control techniques that have been used in receivers which process amplitude shift keyed (ASK) or quadrature phase shift keyed (QPSK) signals. Examples of such techniques include "differentiate and multiply" techniques of the type mentioned above, cross product discriminator techniques, discrete Fourier transform techniques, techniques which employ a bank of all-pass filters to estimate frequency offset of a base band signal having a frame structure which contains a synchronization field, and techniques (commonly employed in analog receivers) which use the principle of frequency counters.
It would be desirable to implement rapid combined timing error and frequency error estimation in a receiver (to enable rapid adjustment for both timing and frequency error). It would be particularly desirable to do so in a robust manner, which functions well in the presence of a variety of transmission channel impairments and in a fast fading environment. The present invention is a method and apparatus for performing such robust, combined timing error and frequency error estimation. The invention can be implemented in a variety of receivers (including receivers which receive and process ASK or QPSK signals). The invention is particularly useful in cellular radio-telephone receivers, although it is also useful in other types of radio telephone receivers and in computer and other data communication systems.